The 92nd mitzvah is that a Nazirite is commanded to let his hair grow [for the duration of his Nazirite vow].

The source of this commandment is G‑d's statement,1 "He must let his hair grow long."

In the words of the Mechilta, "The words,2 'he shall be holy' teach us that he should be in a state of holiness when it grows. The words, 'he must let [his hair] grow long' makes this a positive commandment. What is the source of the prohibition?3 The verse,4 'no cutting instrument shall come upon his head.' "

There it also says, "Who is covered by the positive commandment [and not covered by the prohibition]? One who rubs earth or applies depilatories on his head." The meaning of this statement: a Nazirite who places a depilatory on his head does not transgress a prohibition, since a cutting instrument was not used. He will, however, violate the positive commandment, "he must let his hair grow long," since he did not allow it to grow.

[This counts as a positive commandment because] according to our principles, a prohibition which stems from a positive commandment is counted as a positive commandment.

The details of this mitzvah are explained in a number of passages in tractate Nazir.